1. Field of the invention
The invention relates to the improvement of the frost-de-icing salt-resistance of concrete constructions.
2. Brief Description of the Prior Art
Solidified concrete and mortar always contain a fine capillary pore system formed in the cement paste by hydration and which promotes the water condensation in the constructed body. Due to the capillary system, water is also directly absorbed from atmospheric moisture and not only in liquid form as dew or precipitation.
For a specific pore distribution, the amount of condensate depends on the relative air humidity of the atmosphere.
The maximum depth of water penetration is usually less than 3 cm in all properly manufactured concretes having a water/cement value (W/C) of not more than 0.6.
Beside the gel pores of from 1.5 to 3 nm, with a water content--designated as "solid water"--which can be expelled only at high temperatures, and the capillary pores of from 1 to 10 .mu.m (measured according to Brownyard, T. L., Proc. ACI 43, 1947, 101-132), larger pores and fissures occur in the surface area, depending on the composition of the mixture and on the production quality.
The liquid water which penetrates into the surface layers is converted into the lighter ice, having a density of from 0.88 to 0.92 g/ml, and expands by about 11%. Although part of the water, e.g. that which is present in the narrower capillaries, may still be liquid at, e.g. -10.degree. C., as a result of the lowering of the vapor pressure, the water freezes in the so-called water pocketes, the inner water puddles, water pores and cracks and exerts a strong pressure on the concrete body. The expansion due to the pressure of the ice outweighs the contraction by the cooling-off. If the water is converted into ice also in the narrower capillaries, the change of length may amount to 1 to 2 mm/m at -20.degree. C. However, a concrete which has once been damaged for the first time by freezing cannot revert to its initial condition upon thawing due to the resulting more open inner structure, in particular An the surface area.
The effect of air pores (AP) on concrete was discovered by chance in 1940 in the USA. At first, the occasional good freezing behaviour of concretes was assigned to some specific types of cements. The cements which seemed to be frost-resistant "by nature" were contaminated by a slight amount of lubrication oil which acted as air pores agent (AP agent). (Dyckerhoff, H., ZKG 1, 1948, 93-95).
If 16% AP are present in a concrete, an expansion is not observed when the length is measured during the alternation of frost and thawing but a shrinkage at -20.degree. C. amounting to a some tenths mm/m, whereas the same condition is achieved during thawing, i.e. a change relative to the initial condition does not occur. With 8-10% AP, the cement stone does not change its length between +20.degree. C. and -20.degree. C. (according to T. C. Powers).
The effect of the air pores in the solidified cement paste is based on the fact that they are not filled with water when the concrete becomes wet and are available as volume reserve during the expanding freezing of the capillary and crack water and prevent the hydraulic and ice pressure. Ice and water can escape into the air pores. The size of the AP which exhibit an antifreezing effect is between 0.1 and 2 mm. According to the statements of other authors, the diameter of the AP should not exceed 0.2 mm. The content should be between 4 and 5% and the maximum distance between the air pores 0.25 mm to provide enough auxiliary and quickly accessible room if ice is formed spontaneously,
The positive influence of the air pores on the frost-thaw behaviour is counteracted by a negative influence on the strength characteristics. According to K. Walz (DAfStb. H. 123, 1956), the bending strength is reduced by 2 to 3% for each % of incorporated air and the tensile strenght by 3 to 4%. If one departs from each of the maximum values, which is advisable by way of precaution, a bending strength reduced by 15% and a tensile strength reduced by 20% result form the necessary 5% AP content.
A further advantage of the LP technique is the relatively great variable of the amount to be used. An amount ranging from 0.5 to 2 g/kg cement are recommended. The difficulty of providing the proper dosage of AP agent resides in that the need for AP agents increases with the fineness of the cement, for example from 3000 to 5000 cm.sup.2 /g specific surface, to about the double amount, Water-soluble alkalies of the cement promote the AP formation and the organic substances (0.015%) which are present in all technical cements can increase or reduce the need.
The amount of AP agent determined by preliminary tests must be added in exact amounts when the mixing takes place. In view of the dosage of between 0.5 to 2 g/1000 g cement, this requires due care which is not guaranteed under construction conditions with added amounts of between 0.05 to 0.2%, already small differences, based on the absolute amounts, can have a very strong effect on the quality of the concrete.
The formation of an optimum AP structure additionally depends on the amount and quality of the aggregates. The grain size distribution within the aggregates and in particular the fine grain proportion have here the strongest influence on the size and distribution of the air pores in the Concrete.
It is obvious that the frost damages in road surfaces and landing grounds of airports are constantly increasing although these concretes have been made with the help of AP agents. The most important reasons are the following:
As already described above, it is likely that one has not succeeded so far in creating the optimum AP arrangement and AP size distribution. In practice, it is not the frost-thaw-interaction resistance which is decicive but almost exclusively the effect of de-icing salt during the frost-thaw alternation, to put it briefly, the frost-de-icing salt resistance.
In the winter of 1978/79, about 3 kg salt/m.sup.2 were spread on the highways of the Federal Republic of Germany. The attack on the concrete surface and shortly thereafter on the steel reinforcement is additionally effected by salt solutions of high concentration up to saturated solutions.
If an icy road is thawed by spreading salt, a so-called freezing mixture consisting of salt and ice is first formed, causing a sudden drop of the temperature of the surface layer. In practice, this is called a cold shock.
The salt concentration of the melt water rises during daytime, in particular under the influence of solar radiation, due to the evaporation of water. After sunset, there is a drop in temperature and the solubility for the salt decreases and is further diminished if part of the water is converted into ice, which again increases the salt concentration. The salt crystallization reduces the available pore volume for the expansion of ice and consequently increases the pressure in the concrete surface system. The hydrate formation of the de-icing salts causes a further rise of the inner pressure, all the more since the surface layer is covered with ice and hinders the escape of water and salt. NaCl and NaCl.2H.sub.2 O are at equilibrium in the same amount already at temperatures of about zero degree (exactly at .+-.0.15.degree. C.).
Attempts have been made to hydrophobize a thusly damaged concrete with silanes, siloxanes or polysiloxanes to protect its surface and the steel reinforcement. However, this has not been satisfactory, because the chemical resistance of hydrophobizations is insufficient.
In particular the reactions of the cement-bonded and lime-bonded building materials are important. These substances form new reaction products, even after years, said reactions taking place preferably at the inner surface. New non-hydrophobic surfaces are formed thereby and surfactants adhering to the inner wall of the pores are incorporated into the new reaction products and thus become ineffective.
The second aspect is the reactivity of the building materials vis-a-vis surrounding substances. Due to the carbonatization of alkaline building materials, the pH-value of the pore solution and the composition of the inner surfaces change, which may result in a desorption of the hydropnobizing agent.
A further problem encountered in hydrophobization is the fact that hydrophobized building materials cannot be treated a second time since the hydrophobizing agent (in general) cannot penetrate into already treated pores. Second coats are, therefore, not recommendable in case of impregnations. Redevelopments of formerly hydrophobized constructed bodies are likewise problematic.
The hydrophobization of concrete having steel reinforcement is usually detrimental to the corrosion prevention of the built-in steel. The carbonatization reduces the alkalinity of the concrete and consequently the inhibition of the steel surface. Since the pore openings are not obstructed by liquid water, carbon dioxide, atmospheric oxygen and water vapor can penetrate forward into damaged concrete without any impediment until they reach the iron. A slight adhering moisture on the reinforcement is enough to start and continue the corrosion of the steel.
Thus, there existed the necessity for providing a sufficient frost-de-icing salt resistance to new concrete constructions and for making old buildings likewise resistant to frost and de-icing salts and for additionally protecting the reinforcement against corrosion.